Inductance device



Aug. 27, 1-946. c, JAN VAN LQON 2,406,720

INDUCTANCE DEVICE Filed March 1'7, 1943 Fly.

H 4 r 32 2% 3 "22m g. #4 BY Tatented Aug. 27, 19 46 INDUCTANCE DEVICECarol Jan van Loon, Eindhoven, Netherlands, as-

signor to Hartford National Bank and Trust Company, Hartford, Conn., astrustee Application March 1'7, 1943, Serial No. 479,465 In theNetherlands March 8, 1941 1 Claim. 1

The invention relates to self-induction coils whose self-induction maybe adjusted by a core of magnetic material which is slidable in thefield of the coil, said coils being referred to hereinafter asslidable-core coils. These slidablecore coils are utilized inter alia inradio-receiving sets in which they may form part of the tuning means.

In practice it is often desirable that the value of the self-inductionof the slidable-core coil as a function of the position of the iron coreshould have a predetermined course. Such is the case inter alia insuperheterodyne receivers if in the tuning circuits use is made ofslidable-core coils. As is well-known, with a superheterodyne receiverthere must exist over the whole of the wave range a constant differencein frequency between the tuning of the oscillator circuit and that ofthe preceding circuits so that, given a determined course of the tuningof the preceding circuit, a determined course of the tuning of theoscillator circuit is fixed.

If the tuning is effected by means of a fixed inductance and a variablecondenser, a substantially constant difference in frequency may beobtained by connecting a capacity in series with and a capacity inparallel with the variable condenser. These additional capacities shouldpreferably be formed as trimmers in order to ensure that adjustment ispossible.

If, however, the tuning is effected by means of a variable inductanceand a fixed capacity, as is the case with the use of slidable-corecoils, it would be necessary to try in a similar manner to obtain thedesired result by connecting an additional inductance in series with andan additional inductance in parallel with the variable inductance. Thisleads, however, to an expensive and complicated system of connections.

If these additional inductances are omitted, it is possible to obtainthe correct difference in frequency between the oscillator circuit andthe preceding circuits only at two points of the wave range, for examplein the neighborhood of the highest and of the lowest frequencies; in theintermediate region there is produced in this case, however, aninadmissibly large divergence from the desired difference inwave-length, which divergence may amount in the middle of theintermediate wave-length range up to 60 or 70 kilocycles.

It is, therefore, the main object of the present invention to improvethe tracking relation between the signal frequency and oscillatorcircuits of a superheterodyne receiver so that the correct frequencydifference will be obtained not only at the limits of the respectivefrequency ranges of said circuits but also at one or more intermediatepoints.

In the drawing:

Figs. 1 and 2 are curves showing conditions of tracking and mistrackingbetween a pair of permeability tuned circuits at various positions ofadjustment of the tuning cores. Fig. 3 discloses a known constructionfor effecting inductance variation in coils. Fig. 4 discloses a coil andcore combination, in accordance with the present invention for effectinga predetermined inductance variation. Figs. 4a and 4b arecross-sectional views taken respectively, on the lines a a and b-b ofFig. 4 and Fig. 5 discloses curves showing the improved trackingrelation obtained with the present invention.

Figure 1 shows the course of the divergence d in kilocycles as afunction of the stroke S in millimeters of the slidable core in theabsence of precautionary measures for effecting tracking at anintermediate point.

In Figure 2 the curve I indicates the desired course of theself-induction L of the coil as a function of the stroke of the core inmillimeters whereas the curve 11 indicates the course which reallyoccurs if no particular steps are taken. In order to obtain the desiredcourse it is necessary, as ensues from Figure 2, to increase the slopeof the self-induction variation at the beginning of the stroke and todecrease this slope at the end of the stroke.

According to British patent specification 410,217 it is known toincrease the slope of the variation of the self-induction of a slidablecore by causing a core of copper and a high-frequency iron core toslide, the one after the other, through the coil. A constructionsuitable thereto is shown in Figure 3.

In this figure l represents a coil, 2 is a core of high-frequency ironand 3 is a rod or tube of conductive material, for example, copper. Uponassuming that the initial position is that in which the whole of thecore 3 is within the coil and the whole of the core .2 is outside thecoil, the selfinduction of the coil increases, when both cores arecaused to slide from the right to the left, to a higher extent thanwould be the case with the use of a slidable core since the iron coreexerts its action in a coil portion which steadily increases and, inaddition, the copper core 3, which acts as a short-circuited winding,encloses a steadily decreasing number of lines of force. Since, however,by utilizing this construction it aicavao is possible to increase theslope of the increase of the self-induction but impossible to decreasethis slope, the use of this form of construction cannot give here thedesired result.

According to the present invention, in order to obtain withslidable-core coils a predetermined course of the self-induction as afunction of the stroke of the core there is provided a body ofconductive material which is caused to slide, jointly with the core,into the field of the coil, the shape of the body of conductive materialbeing so chosen that the desired course of the self-induction isobtained.

In one form of construction according to the invention the body ofconductive material is preferably provided on the core itself or, if thecore is made of pressed material, it is pressed jointly with the core toform a mechanic unit.

The invention will be explained more fully with reference to Figures land 5 of the accompanying drawing.

Figure 4 shows a slidable-core coil of cylindrical shape wherein thecoil body is denoted by I, the turns of the coil by 2 and the slidablecore by In Figure 5 the course of the self-induction L as a function ofthe stroke of the slidable core is shown by curve I.

If for some reason or other it is desired that the curve indicating thecourse of the self-induction should have a more horizontal course at thebeginnin and at the end but a steeper course in the middle, this may beachieved, in accordance with the invention, by providing on the core 3 abody or insert 4 of conductive material which has a determined shape.The conductive material may be for example, copper. The shape of thisbody or insert may be determined experimentally and may have in adetermined case the shape indicated in Figure 4 which diminishes incross-section from each end inwardly toward the center.

Otherwise, in constructing such a slidable-core coil the fact has to betaken into account that the presence of the copper body 4 necessitatesan increase of the number of turns or again an increase of thepermeability of the slidable core in order to be able to obtain the samevalue of the maximum self-induction. Care should be taken in this caseto ensure that the minimum value of the self-induction does notincrease, which may be effected by increasing the permeability of thecore. In the absence of the copper strip according to invention thecourse of the self-induction curve would become steeper over the wholeof the range; by providing the copper body the course of the curve onthe desired portions is restored to the desired slope.

What I claim is:

A. variable inductance device comprising a coil of the solenoid type, acore movable axially within th coil and adapted upon movement from aposition outside the coil to a position fully contained within the coilto vary the inductance of the coil in a predetermined manner as afunction of the coretravel, said core consisting of a substantiallycylindrical body of magnetic material and an insert of non-magneticconductive material embedded in the surface of said body, the surface ofthe insert being coextensive with that the body, and said insertdiminishing in cross-section from each end inwardly toward the center.

CAREL JAN VAN LOON.

